MIL-DTL-17361G(SH)
c. For fault currents above the interrupting ability of the circuit breaker, the time-current characteristics of the
fuses shall be such as to provide adequate back-up protection to the circuit breaker on fault currents up to 100,000
amperes AC asymmetrical. Fuses shall clear the fault prior to any damage to the circuit breaker. After clearing a
fault, the unblown fuses shall perform their current carrying and interrupting functions.
d.
It shall not be possible to install the wrong type current limiting fuses in a specific circuit breaker type.
e. Mounting of the fuses in a fuse unit located at the load terminals shall be such that blown fuses may be
replaced quickly without removing the circuit breaker housing cover and without danger to operating personnel.
f.
The circuit breaker shall trip automatically when an attempt is made to remove the fuses with their housing.
g.
Upon blowing of one or more fuses, the circuit breaker mechanism shall open automatically.
h. The circuit breaker shall not operate to the closed position when one or more fuses are blown unless the
damaged fuse or fuses have been replaced.
3.5.17.2 Current limiting fuse units. When specified (see 3.1 and 6.2), current limiting fuse units shall be
provided with back-connected removable mountings. Units shall contain three separate nonremovable current
limiting fuse elements to provide short circuit protection at high current values. Fuse unit shall be in accordance
with MIL-PRF-15160.
3.5.17.2.1 Function of current limiting fuse units. Current limiting fuse units, when connected to the circuit
breaker load terminals and the load, shall be readily inserted or removed from their mounting base/block in the field.
In addition to interrupting the circuit, each fuse unit shall include mechanical blown fuse element indicators. The
blowing of the current limiting fuse unit types under short circuit conditions shall occur within the rms asymmetrical
ampere values specified (see 3.1).
3.5.17.2.2 Current limiting fuse unit characteristics. The combination of the current limiting fuse unit and the
circuit breaker shall have the following characteristics:
a. Time-current characteristics of the fuse unit and the circuit breaker shall be coordinated to prevent
unnecessary blowing of the fuses and to provide protection to the circuit breaker without damage.
b. The circuit breaker shall safely and satisfactorily clear faults up to the current values at which the fuse unit
will blow (see 3.5.17.2.1). During the circuit breaker performance, the fuse unit shall not blow, be damaged, or
change its characteristics.
c. For fault currents above the interrupting ability of the circuit breaker, the time-current characteristics of the
fuse unit shall be such as to provide adequate back-up protection to the circuit breaker on fault currents up to
100,000 amperes AC asymmetrical. The fuse unit shall clear the fault prior to any damage to the circuit breaker.
d. After installation, it shall not be possible to insert a current limiting fuse unit type of a higher crossover
current rating into a fuse unit mounting base/block installed to accommodate a fuse unit type which-has a lower
crossover current rating.
e. Mounting of the fuse unit shall be such that a blown fuse unit may be replaced quickly without removing
the circuit breaker housing or switchboard front panel and without subjecting operating personnel to danger.
3.5.17.3 Integral (fuseless) current limiting devices. Circuit breakers with integral (fuseless) current limiting
devices shall interrupt 100,000 amperes AC asymmetrical or more (see 3.1). In addition to interrupting the circuit,
integral current limiting devices shall trip the circuit breaker mechanism. The current limiting device shall permit
restoration of service by resetting, if necessary, and returning to the "on" position without replacement of any of its
parts or elements.
3.5.18 Mounting bases/blocks, mounting arrangements, and circuit breaker/mounting base/block interface
connections.
3.5.18.1 Mounting base/block. When specified (see 3.1 and 6.2), mounting bases/blocks shall be provided.
Mounting bases/blocks shall be of a molded insulating material as specified in MIL-E-917 with associated metallic
parts as specified in 3.1 and shall not cause failure of dielectric strength and insulation resistance requirements of
3.5.14.2 and 3.5.14.3, respectively.
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